3.3.1. Nucleogenesis Differences

Relative abundances are usually not compared in the absolute, but generally compared along some baseline. For example, C/N or N/O may be studied as a function of metallicity. Metallicity is generally held to be O/H for nebular astronomers and Fe/H for stellar astronomers. In both cases (O and Fe) the elements are thought to be predominantly primary, i.e., built up chiefly from the hydrogen that was contained in the protostar during its initial collapse.

A secondary element is one that is built up from some other element besides H. For example, the C-N-O bi-cycle process that converts H into He in intermediate mass stars converts most of the C and O into N (since the ¹⁴N(p, ) ¹⁵O reaction is the rate determining step). When this reaction chain has reached equilibrium, the amount of N produced will be proportional to the metallicity of the star. Thus, when plotted versus metallicity, ratios of secondary elements to primary elements (e.g., N/O or N/C) would be expected to increase linearly proportional to metallicity. This is in contrast to ratios of primary elements (e.g., Fe/O or C/O) which would be expected to be independent of metallicity.